Measurement of weak light levels is a common procedure in science and technology.
One of the most sensitive photodetectors is a photomultiplier tube (PMT), or just photomultiplier. The basic structure of this device is a vacuum tube containing a light sensitive photocathode and an electron multiplier structure. Electric field by a high voltage is applied over the system. Photons to be detected hit the photocathode from which they liberate photoelectrons by a photoemission process. The electron multiplier consists of a series (typically 6-16) of secondary emission electrodes, called dynodes, with rising electric potential arranged between them. The photoelectrons from the cathode are directed to the first dynode where they produce several secondary electrons which are, in turn, directed to the next dynode where secondary emission is repeated, and so on. This results in amplification so that the signal from the output electrode, anode, is high enough to be handled electronically. Disadvantages of photomultiplier tubes are relatively high cost and need for high voltage which limit and complicate their versatility.
A different class are various semiconductor photodetectors, e.g. photodiodes, phototransistors and charge coupled devices (CCDs). Common to them is that light is allowed to affect a semiconductor material, where it generates charge carriers (electrons and holes) that are collected to produce an electrical signal. A problem with semiconductor detectors is that the carriers have to migrate in the bulk of semiconductor material where thermal energy produces a high background noise.